1. Field of the Invention
The present invention generally relates to a control apparatus for a chopper circuit. In particular, the invention concerns a chopper control apparatus which is capable of detecting commutation failure occurring in operation of the chopper with the aid of a digital computer such as a micro-computer capable of performing arithmetic operations.
2. Description of the Prior Art
In a chopper control apparatus for a d.c. motor, it is generally conceivable as a control method employing a micro-computer that an interrupt signal is generated for every operation period of the chopper to thereby allow information such as motor current, command values and the like to be fetched by the micro-computer for arithmetically determining a required conduction period or duty factor of the chopper operation for the succeeding period during the current or instant period of the chopper operation.
In this connection, in the case of a chopper control apparatus for use in a battery-powered fork lift truck, for example, it is required that the chopper control apparatus be implemented to be inexpensive in cost to be small in size so as to occupy less space for installation. Further, provision of plural micro-computers is not preferred for practical applications from the economical standpoint. To meet these requirements, it is necessary to use a single micro-computer effectively and efficiently so that it can satisfactorily process a number of various functions such as, for example, control for short-circuiting the chopper by means of a switch in a range where a longer conduction period of the chopper is required, limitation control of duty factor of the chopper for preventing the speed of a fork lift truck from exceeding a predetermined value when the fork carrying a load thereon has been moved up, demand of the operater for alterating the duty factor limitation and others. Under the circumstances, it is required that detection of failures and in particular detection of the commutation failure of the chopper be dealt with through software-based processing while the time required for the arithmetic operation being reduced to a possible minimum. For the detection of the commutation failure of the chopper under the control of the micro-computer, it is generally conceivable that detection of the commutation failure itself is realized on the hardware base, while various processings as required are carried out on the software base by producing an interrupt request signal allotted with a high priority upon detection of the commutation failure. However, this scheme will require not only a complicated configuration of hardware but also additional provision of a circuit for generating the interrupt request signal of the higher priority.
As another method of detecting the commutation failure of the chopper circuit, the terminal voltage (or current) of the chopper is fetched into the micro-computer after the lapse of the commutation period in succession to the generation of the turn-off signal of the chopper, to thereby determine the occurrence of the commutation failure on the basis of the presence or absence of the terminal voltage (or the magnitude of the chopper current). A commutation failure detecting method based on this principle is disclosed, for example, in Japanese patent publication No. 4187/1982 (Laid-open No. 106420/1978). However, this system requires delay means for detecting the elapse of commutation period and means for newly producing the interrupt request signal of higher priority in response to the output signal from the delay means.
By the way, the chopper control apparatus must be, as a matter of fact, so constructed that probability of occurrence of the commutation failure is extremely low, so far as the turn-off signal is normally produced. Accordingly, the commutation failure may occur only when the turn-off signal is not produced normally for some cause such as failure in the hardware structure or when the chopper is erroneously fired due to a noise or the like, whereby a load current of a magnitude exceeding the commutation capability of the chopper is caused to flow. Thus, according to the method of detecting the commutation failure after generation of the turn-off signal as mentioned above, it is impossible or requires unreasonably great delay to detect the commutation failure brought about by the false firing or malfiring in the state in which a voltage is applied across the chopper while no gate signal is applied or the commutation failures ascribable to various other false or erroneous operations.